FRANKFURT (Reuters) - Greater Gabbard Offshore Winds has signed a preliminary agreement to buy wind turbines from Siemens for an undisclosed sum for its wind farm off the British coast, the German group said on Thursday.

Siemens said in a statement the deal, the largest ever for offshore wind turbines, was for 140 Siemens 3.6 megawatts turbines for delivery in 2009 and 2010.

The agreement is preliminary because Greater Gabbard, a joint venture between Ireland's Airtricity and U.S.- based Fluor, is still finalizing their project finance.

"There is a global shortage of wind turbines and ever increasing competition to secure their supply. In this context, to have landed this agreement, is a major coup," Airtricity Chief Executive Eddie O'Connor said in the statement.

The deal followed UK government planning permission for a 500-megawatt wind farm off Suffolk, eastern England that could avoid emitting nearly 1.5 million tonnes of carbon dioxide a year by displacing fossil fuel-fired generation.

The wind turbines will be sited around the Inner Gabbard and Galloper sandbanks. It will be the first UK offshore wind farm to be built outside territorial waters and will provide electricity for more than 415,000 homes.

Construction of the project will commence in mid-2009 with completion expected by the end of 2010.

Patrick Flaherty, Fluor's managing director, said securing turbine supply was a critical milestone as it gave confidence to all stakeholders as to the viability of the project.

"We will continue to work with Airtricity to advance the project to financial close in early 2008," he said.

Britain has some of the windiest weather in Europe and is trying to cut its carbon emissions by building offshore windfarms to harness that power.